Some Questions on Inspection n Testing! Please Help!

nazoom · Nov 25, 2010

I have some questions with answers but not very sure if they are correct. Please help and be patient with. I am still a newbie. I have the books OSG, GN3, and 17 wiring edition.

Q.1 Describe the methods for verifying voltage drop compliance with BS7671 in terms of (a) measuring circuits impedance and (b) calcification. (3.31)

Answer: For (a) measuring the impedance ? Don’t know! Please Help.

For (b) I know the answer and the formulae

Q.2 Explain the importance of selecting protective devices appropriate to the prospective fault current. (3.29)

Answer:
Devices used for providing PFC are:
a) Semi enclosed Fuse BS3036
b) Cartridge Fuse BS1361
c)Cicuirt Breaker BS 60898
d) RCBOs
And The breaking capacities rating of the protective devices shall be greater than the Prospective fault current and prospective short current.

Are these answers correct?

Q.3Given maximum tabulated values of Zs, verify that measured values are acceptable, taking into account conductors operating and ambient temperature.(Q3.25)

Answer:
1.For TN-C-S systems maximum Ze is 0.35 ohms
For TN-S system, the maximum Ze is 0.8 ohms
For TT system Rcd is required

2. The ambient temperature must not exceed 30C
3. The cable conductors is made of copper.
4. For other than lighting circuit (voltage drop no more than 5 %) the voltage drop should b be no more than 5 %
5. A disconnection time of 0.4 Sec in circuit of 32 A.

Are theses answers correct?

Any help is much appreciated.

IQ Electrical · Nov 25, 2010

Q1
The methods of verifying volt drop compliance are measurement and calculation-you can actually measure the difference between the supply voltage and the voltage at the end of the circuit in question. For calculation, you can use the usual formulae.

Q2
It doesn't ask you to list devices, merely to explain the importance of selection. Don't mention PSSC or PEFC, the question uses the term 'prospective fault current' (the higher of the 2 values) so stick to that.
The short circuit capacity of the device should be greater or equal to the PFC value at the point which the device is required to operate.

Q3
You've listed maximum recommended Ze values from ESQCR 2002 but the question is referring to Zs and is looking for an explanation of the 'rule of thumb' 80% comparison values for measured Zs.
The volt drop percentages and disconnection times are irrelevant to the question.

nazoom · Dec 2, 2010

Thanx for the help. I am still unsure of the answer could you please explain more or give more details or clues to where i can find the answer any links or pages from books. Sorry I am a newbie and having hard time understanding all this!

So for Q.2 What are the importance of selecting protective devices appropriate to the prospective fault current?

Answer:
The devise providing protection against fault current only shall be installed where overload protection is achieved by other means. A devise shall be capable of breaking, and for circuit breaker making, the fault current up to and including the prospective fault current. such a devise shall satisfy the requirement as example for the devises:

i) A circuit breaker with short circuit release
ii) A Fuse.

A devise providing protection against fault current shall be installed at a point where there is a reduction in the c.s.a, or other causes reducing the current carrying capacities of conductor of installation.

Q.3
Answer:
Maximum Zs for circuits with protective conductor having 1 mm to 16 mm c.s.a., At temperature 70 C degree, the over-current protective devise (ie. Fuse) to BS 88 part2 is for 0.4 s highest is 6.8 ohms and for 5 s the maximum Zs is 2.3 ohms, For BS 1361 maximum Zs for 0.4 Sec is 8.4 ohms and for 5 sec the maximum Zs is 2.3 ohms. finally for BS 3036 for 0,4 sec the maximum Zs is 7.7 ohms, and for 5 sec the maximum Zs is 3.1 ohms.
Is this correct?

Thanx a million for the help.

IQ Electrical · Dec 2, 2010

Q2 Model answer is:

The breaking capacity rating of each protective device shall be not less than the prospective fault current at it's point of installation (434.5.1).

Q3 Model answer is:

As a rule of thumb, the measured value of earth fault loop impedance for each circuit at the most remote outlet should not exceed 0.8 of the relevant value in the BS7671 tables.
This takes into account the increase of resistance of the conductors with the increase of temperature due to load current and errs on the side of safety.